Errors at penetrations

Penetrations in the field of the roof are potential locations for air leakage, thermal loss, and condensation. Typical roofing details often do not specifically address air sealing, and standard details typically show the insulation butting up to a vertical penetration, such as a pipe. Rigid insulation should be held back a minimum of 1-inch from the penetration and closed-cell spray foam should be installed between the rigid insulation and the penetration. The addition of spray foam, which is not typical to roof details, creates an effective air seal at penetrations. It is important to select pipe penetration flashings that can be installed airtight, such as with the use of a split pipe boot. Often pipe insulation is needed in addition to the roof insulation. The location and installation of the pipe insulation should be coordinated as pipes, such as for cooling equipment, can introduce condensation into the roofing system if not insulated properly.

Errors at curbs

Curbs, such as for mechanical equipment, are also potential locations for air leakage and resulting condensation. Again, proper air sealing at curbs is important for long-term performance. Similar to the detail for penetrations, rigid insulation should be held back a minimum of 1-inch from the curb and closed-cell spray foam should be installed between the rigid insulation and the curb. The addition of spray foam makes an effective air seal at penetrations. However, mechanical units often sit on steel framing, which is uninsulated and creates a thermal bridge within the roof assembly. The steel framing should be insulated as well; and if there is a steel deck, the deck flutes should be filled with spray foam.

Additionally, during construction, the membrane base attachment method should be discussed with the roofing manufacturer. In normal roofing installations, fasteners are installed around the perimeter of the curbs and penetrations to secure the field sheet of the membrane. This creates additional thermal bridges as the fasteners must span from the top of the membrane down through the steel deck. This allows for air and thermal transfer through the roof assembly. An alternate method would be to secure one piece of wood blocking around the perimeter of the curb to the deck, then install insulation to the designed height, and install the fastener into the blocking, not the steel deck.

Challenges for winter construction

During new construction of a cold storage building, construction-generated moisture can move up into the roof system. This moisture can come from curing of concrete floors and concrete roof decks, the use of propane heaters, or ambient humidity, rain, or snow. If the moisture becomes trapped in the roof system, the bottom portions of the insulation can freeze during temperature pulldown or regular operation. This concern may warrant the use of a vapor retarder at the deck level to keep construction-generated moisture from migrating into the roof system. Selecting an air barrier that is vapor open, such as a Class III or higher vapor retarder, will allow some drying potential during operation of the building while preventing moisture-laden air from getting driven into the roof system during construction.

When moisture laden air is allowed to enter into the roof assembly, the materials can become frozen. Frozen insulation results in an effective R-value of zero, which affects overall energy efficiency of the cold storage facility. Frozen insulation also creates reroofing issues when trying to remove existing insulation from the facility as it most often becomes frozen to the deck substrate and can be quite disruptive to the continued operation of the facility.

Temperature Pulldown

Temperature Pulldown, or Temperature Draw Down, raises another challenge for the cold storage design professional. ASHRAE offers the following temperature pulldown considerations for cold storage buildings in the 2018 edition of the ASHRAE Handbook—Refrigeration:

Chapter 24:
“The first stage of temperature reduction should be from ambient temperature down to 35F, at whatever rate of reduction the refrigeration system can achieve. The room should then be held at that temperature until it is dry.

The concrete slab will contract during pulldown, causing slab and wall joints, contraction joints, and other construction joints to open. At the end of the holding period, any necessary caulking should be done. An average time for drying is 72 hours. However, indicators that may be used include watching the rate of frost formation on the coils or measuring the rate of moisture removal by capturing condensation during defrost.

After the refrigerated room is dry, the temperature can then be reduced again at whatever rate the refrigeration equipment can achieve until the operating temperature is reached. Rates of 10F per day have been used in the past, but if care has been taken to remove all the construction moisture in the previous steps, faster rates are possible without damage.”

The time required for some cold storage facilities to reach the design temperatures could be several weeks. It is important for all finishes within the facility, including roofing, to adhere to the recommended timeline to reach the design temperatures. It is, however, normal to have some contraction of roofing elements since construction materials tend to contract in cold temperatures. It is possible that flashings at penetrations, or termination bars on exterior walls could have slightly shifted during the draw down period. It is important for the roofing contractor to visit the site after the draw down period is complete to re-seal any locations that may have become compromised.

Conclusion

The equation of operations, high-performance roof design, and energy use all hinge on controlling inputs from internal temperature, thermodynamics, moisture risks, vapor control materials, and air leakage. When it comes to designing the roof of a cold storage facility, the details are critical to prevent air and vapor transmission. Cold storage roofing, even more than most construction, requires correct design, quality materials, good workmanship, and close supervision during installation. Design should ensure that proper installation can be accomplished under various adverse jobsite conditions and that materials are compatible with each other. Ensuring an uninterrupted, continuous building enclosure provides a shield that protects both the building’s performance and its longevity.

References

¹ May 2011. Impact of Air Leakage on Hygrothermal and Energy Performance of Buildings in North America. National Research Council of Canada. NRC Institute for Research in Construction. 13th Canadian Conference on Building Science and Technology. 1-2. NRC-IRC-21657.

Kristin M. Westover, PE, LEED AP O+M, is a Technical Manager of Specialty Installations for low-slope commercial roofing systems at GAF. She specializes in cold storage roofing assemblies where she provides insight, education, and best practices as it relates to cold storage roofing. Kristin is part of the Building and Roofing Science Team where she works with designers on all types of low-slope roofing projects to review project design considerations so designers can make informed roof assembly decisions.